In a large saucepan, combine the stock or broth, carrots, celery, curry powder and pepper.
Cover and bring to a boil over medium-high heat.
Stir in the rice.
Return to a boil, then reduce the heat.
Cover and simmer for 15 minutes, stirring occasionally.
Stir in the chicken.
Return to a boil, then reduce the heat.
Cover and simmer for 15 minutes, stirring occasionally.
Stir in the mushrooms.
Cover and simmer for 5 minutes more.
Stir in the buttermilk.
Cook and stir about 1 minute or just until heated through.